Growth of the paleo-Orinoco shelf-margin prism; process regimes, delta evolution, and sediment budget beyond the shelf edge

The first sedimentological characterization and correlation of onshore outcrop and offshore subsurface data (southern Columbus Basin) are presented for the paleo-Orinoco (Upper Miocene-Pliocene) shelf-margin prism, Trinidad Island, Trinidad and Tobago. The paleo-Orinoco River delta system and associated continental slope, which generated the 10-km-thick sedimentary prism, was a mixed river, tide, wave, and sediment-gravity flow system that tracked down to deep-water submarine fans. The analysis here includes: (1) an evaluation of delta-plain to deep-water turbidite sedimentary facies, which are seen in spectacular outcrops along the southwest, south, and southeast coasts of Trinidad Island, (2) well-log correlation of the same Upper Miocene-Pliocene strata across southern Trinidad and out to the southern part of the offshore Columbus Basin along an off-axis transect, because the main fairway into the most rapidly subsiding part of Columbus Basin is structurally complex, and few detailed data have been released for publication; and (3) use of published seismic data for reconstruction of clinoform morphology across the relatively undeformed segment of the margin, with quantitative sediment flux calculations and predictions. Paleo-Orinoco shelf-margin growth was generated by repeated ( approximately <100 k.y. time scale) cross-shelf, regressive-transgressive transits (>100 km) of the Orinoco delta system, with internal variability in clinoform architecture and process-regime changes during shelf-margin construction. The Upper Miocene-Pliocene prism, with its linkage to the southern Columbus Basin and Columbus Channel, is composed of four progradational clastic wedges, each with a thickness up to 2 km, separated by well-known, Trinidad-wide marine flooding intervals of similar extent, followed by an aggradational wedge with decreased clinothem thickness. The studied sedimentary prism spans an approximately 4 m.y. period and shows an overall rising shelf-edge trajectory, despite an overall and periodic falling eustatic sea level during this global icehouse period. The relationships among the Orinoco paleoflux, shelf-edge trajectory, and clinoform height and shape are discussed for a southeastern segment of the margin that is much less deformed than the main growth-faulted Columbus Basin. However, this off-axis segment was also subject to severe tectonic subsidence, as well as high-frequency sea-level, climate, and sediment supply changes. The estimated sediment flux percentage at the shelf edge shows a similar time trend to that of the well-imaged shelf-edge trajectory. The latter shows three long-term ( approximately 0.5 m.y.) trajectory changes, whereas the estimated sediment flux percentage at the shelf edge shows three supply pulses (with shelf-edge flux varying between 40% and 91% of original sediment discharge) through the early Pliocene, with nearly 76% of the sediment on average transported beyond the shelf edge to the deep-water areas, despite a relatively wide shelf. This sediment flux calculation, and particularly the occurrence of flat segments along the shelf-edge trajectory, predicts significant sediment bypass to slope or basin-floor fans on or beyond the toesets of the paleo-Orinoco shelf margin. At short time scales, the variation in the percentage of sediment budget bypassing the shelf edge depends on a series of shelf parameters. However, at geologic time scale, the total sediment flux bypassing the shelf edge is two thirds to three quarters of the total sediment budget.